[pulseaudio-discuss] Volume problems in Pulse and alsamixer.

[Jud Craft]

Noted.  The method for attaining base and shifting the dB scale makes
good sense.

On Sat, May 23, 2009 at 7:08 PM, Lennart Poettering
<lennart at poettering.net> wrote:
> On Sat, 23.05.09 18:42, Jud Craft (craftjml at gmail.com) wrote:
>
>>
>> Hmm.  I assume that when you say that ALSA gives no reference point
>> for 0dB on a card, this is because the cards themselves have no such
>> reference point, correct?
>
> The ALSA drivers simply expose the dB information that is documented
> in the hardware documentation. Often that documentation mentions
> relative to which reference level those dB values are. However, ALSA
> doesn't encode that information in any way. Also, a lot of
> hardware/documentation does *not* document the reference level, for
> example the USB spec doesn't do that at all.
>
> We had longer discussions on alsa-devel about this. Ideally all
> 'inner' mixer sliders set 0dB to 'same amplitude on input as on output
> of the element in question'. And all 'outer' mixer sliders set 0dB to
> some defined reference voltage. However, realisitically it is an
> incredible amount of work if not impossible to do that. For many cards
> (especially cheaper ones) we'd have to physically measure the output
> voltage. We'd then have to create a huge quirk database that includes
> all that measured data. And then, what would we do about integrated
> speakers (i.e. speakers with an USB cable)? Measure sound pressure?
>
> Given the limited manpower, the lacking equipment and the fact that we
> have better things to do then spending all days with measuring sound
> cards this is unlikely to be added.
>
> But of course, you are welcome to contribute work in this area!
>
>> Next question, for the curious:  how does PA know how to shift the dB
>> scale to where 0dB matches maximum amplification, if ALSA itself
>> doesn't know?
>
> For a specific driver ALSA tells us that the volume control range goes
> from (as an example) -64dB to +7dB. We have no clue what 0dB means in
> this range data. However, what we know is that we can shift this to
> -71dB to 0dB and then we know that 0dB is max hw amplification, -71dB
> is min hw attenutation and -7dB is the 'base' volume.
>
> Of course, this shifting doesn't add any information we didn't have
> before. However it unifies behaviour across different sound cards a
> bit and also makes the mixer a bit more like your stereo where 0dB is
> max amplification too.
>
>> So what you've said is basically -- "Yes, we know this is weird, but
>> we have a new volume scaling formula in the current tree that should
>> help balance out your volume control a little bit."  Right?
>
> What I said is basically that it is *not* weird and *does* make a lot
> of sense, although the 'percent' to loudness mapping in 0.9.15 can be
> improved. Which I did in PA git. And that you should stick to the dB
> information when comparing volume slider settings, and ignore the
> 'percentage' data, because it is very arbitrarily mapped.
>
> Lennart
>
> --
> Lennart Poettering                        Red Hat, Inc.
> lennart [at] poettering [dot] net
> http://0pointer.net/lennart/           GnuPG 0x1A015CC4
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